1. Field of the Invention
The present invention relates to a DC/DC voltage converter for converting a DC voltage into a stepped-up or stepped-down DC voltage, and a voltage conversion control method for a DC/DC voltage converter.
2. Description of the Related Art
A device for performing DC/DC voltage conversion has hitherto been used, which utilizes a switch-ON and switch-OFF operation of switch elements connected in series to perform a combination, of an operation of storing and discharging energy to and from an inductor and an operation of charging and discharging an energy transfer capacitor (see, for example, Japanese Patent Application Laid-open No. Hei 11-186478, and Mitsubishi Denki Giho, Vol. 61, No. 2, 1987).
This DC/DC voltage converter has a circuit configuration in which at least four power devices, each including a switch element and a rectifier element connected in anti-parallel thereto, are connected in series. Only half of the series-connected power devices simultaneously become the ON state (the switch element is switched ON or the rectifier element is connected in forward bias), and hence the breakdown voltages of the individual power devices can be set to be low. It is therefore possible to suppress an increase in conduction loss of each power device caused by a high breakdown voltage and to set a high handling voltage of the DC/DC voltage converter.
By the way, a DC/DC voltage converter for converting a voltage obtained by rectifying a commercial AC power supply voltage (AC 100 V or AC 200 V) or a DC/DC voltage converter for converting a voltage of approximately 100 V to 1,000 V typically includes a power device in which an insulated gate bipolar transistor (IGBT) made of silicon (Si) is used as a switch element and a PiN diode similarly made of Si is used as a rectifier element.
This type of DC/DC voltage converters may constitute a system in a combination with an inverter for converting DC into AC. Examples of the system include an electric drive system for a hybrid vehicle or an electric vehicle, a power conversion system for solar power generation, and a power conversion system for an air conditioner or the like.
The DC/DC voltage converters to be used in those systems are designed to adjust a voltage conversion ratio in accordance with the state of a power supply (such as the irradiation amount of light on a solar battery of the solar power generation system) or the state of a load (such as the rpm of a motor of the electric drive system for a hybrid vehicle), to thereby control an output voltage of the DC/DC voltage converter.
The above-mentioned conventional DC/DC voltage converter, however, has a problem in that the polarity of an inductor current IL may be only one of positive and negative or remain one of positive and negative depending on the change in load amount, which causes a discontinuous power loss of a power semiconductor unit of the DC/DC voltage converter, with the result that transient voltage fluctuations may occur in the DC/DC conversion and the performance may deteriorate.